1. Field of the Invention
The present invention relates to a method for capturing a target substance by means of fine magnetic particles and an apparatus using the method.
2. Related Background Art
In a sensor for detecting a biological substance, such as a protein and a nucleic acid, as well as a chemical substance, such as a gas, the surface area of a reaction site is a critical factor for controlling a detection sensitivity and a reaction rate. An increase in surface area leads to an improvement in the performance of a sensor. Until now, methods using a porous material as a base have been proposed for holding a large amount of reactive substance capable of reacting with a detection target. Such known methods for improving the performance of a sensor employ a microfilter formed of a nitrocellulose film or a nylon film, paper, nonwoven cloth and thread as a base. A porous material, since it has a substantial surface area that is a thousand to ten thousand times as large as an apparent surface area, can immobilize or carry a large amount of component capable of capturing a target substance on the surface. Japanese Patent Application Laid-Open Nos. 2000-1565 and 2000-2705 disclose a method of manufacturing a porous surface body. Furthermore, Japanese Patent Application Laid-Open No. H9-274032 proposes a miniaturized highly sensitive monitoring system for nitrogen dioxide concentration using a porous body.
However, when the surface area of a porous body is increased to detect a trace of constituents selectively or to increase the sensitivity, it is usually not easy to wash efficiently the entire porous body during a washing step carried out after a capturing reaction. In removing a non-target substance and the like, which are substances other than a target substance, by washing, if the amount of such a non-target substance remaining in a porous body is negligibly small compared to that of a target substance, no problems arise. However, if the amount of a target substance in a sample is extremely small, such a substance, which affects the detection of a target substance, must be washed off at a high removal rate. Furthermore, when a porous body having a flow resistance within a practical range of values is placed in a microchannel, the porous body itself acts as a light scatterer. Therefore, such a porous body is sometimes not suitable for optical detection. On the other hand, a porous material used in a gas sensor has very small pores with diameters on the order of nanometers. Such a porous body has problems with permeability and reactivity when used in a solution system, and with flow resistance when used in a microchannel.